1. Field of the Invention
The present invention relates to an antenna system for use in mobiles such as motorcars and other vehicles and particularly to such an antenna system that is suitable for tracking dependent upon the moving direction of the mobile.
2. Description of the Prior Art
With rapid progress of electronic communication techniques, radiowave communication has been popular in various fields. Particularly, with miniaturization of electronic instruments such as transmitter-receivers and others, the spotlight of attention is now focused upon mobile communication using a land mobile telephone or the like.
There is known a cellular mobile telephone system which includes a plurality of ground base stations. Each of the base stations controls the communication link between the base station and mobiles within one area. This system has been adopted in land mobile telephones and the like. However, such a communication system utilizing the ground base stations can only be used in the limited area since the number of base stations cannot infinitely be increased.
Another mobile communication system is also known which utilizes a communication satellite. The mobile satellite communication system is being studied into practical use in various applications since it does not have the aforementioned limitation as in the mobile communication utilizing the ground base stations and can do high-quality services over a wide area of a nation-wide scale.
In the latter case, an antenna to be mounted on the mobile becomes one of very important factors. If the antenna cannot operate well on transmission and reception, a transmitter receiver and associated electronic components cannot function well even though they are very high in performance.
As a mobile such as a motorcar or other vehicle is moving, the direction of the satellite will vary every moment. Therefore, the beam direction of an antenna mounted on the mobile must be pointed to the satellite by use of any suitable tracking means.
A step track method is popular as tracking methods. The step track method is adapted to maintain the beam direction to the satellite by slightly moving the direction of the antenna at a suitable time interval so that the beam of antenna is pointed in the direction of a received signal.
In such mobiles as ships and aircrafts which do not vary in direction very well and in which the blocking effect by any obstruction does not rise, the step track method is satisfactory on tracking the satellite.
However, land mobiles are frequently steered and turned with higher speeds than those of the ships and aircrafts and radiowave from the satellite may be blocked by any obstruction such as building or the like. Therefore, it is frequent that the step track method is not satisfactory in tracking. Once a radiowave is blocked by a utility pole or building, the mobile may miss the satellite completely.
Even if radiowaves are being stably received by the mobile, the strength of received signal may vary more than necessary since the beam direction of the antenna is always changed slightly every moment to search the maximum strength of received signal.
The antenna must be as small and thin as possible since it should be mounted on the mobile. And also, the antenna must provide a low air resistance when the mobile is running.
Mechanically steered antenna cannot be miniaturize since it includes a mechanical drive.
A phased array antenna is known which can be electronically steered. Such a phased array antenna is suitable for use in radar system and mobile satellite communication. It is however difficult to miniaturize the entire phased array antenna because it requires feeding circuits including phase shifters, power dividers feeding and others; control circuits for the phase shifters; and so on, in order to control the atenna beam.
One of small antennas is a microstrip antenna which may be utilized as an antenna element in an array antenna. However, the microstrip antenna has a disadvantage that it has a narrow band width. In order to overcome such a problem, there is considered a stacked microstrip antenna to which a passive element is added to increase the band width. To obtain the band width of 8%, the stacked microstrip antenna requires its height equal to about 0.075 wavelength. When the central frequency is 1600 MHz, it is required that the height of the antenna is about 14 mm. This is too high for the intended purpose. As the antenna element is higher, the mutual coupling is increased. As the result, it cannot perform its function sufficiently in the gain and the axial ratio.